2012 BMW Z4 SDrive28i: Dyno Tested

It's true, BMW is phasing out its traditional 3.0-liter inline-six and replacing it with a turbocharged four-cylinder. Across the board, no exceptions, the aspro mill will soon be gone  -- finis, stick a fork in it.

The 2012 BMW Z4 SDrive28i is the first to receive the new four-pot heart, alongside the 2012 528i. We've only driven the sedan at a site far from our slide rules and abaci, but we did recently nab a Z4. After running its performance numbers at the test track, we did the right thing and strapped the Z4 to the dyno rollers.

Better get used to the idea of boosted Bimmers, because you won't find a normally aspirated BMW after 2013.

The idea of this downsizing and boosting is to bolster fuel economy, but anyone that's been around turbos knows that where there's boost, there's torque, too. BMW rates the N20 power plant in the 2012 Z4 thusly -- 258 lb-ft from 1250 to 4800 rpm, and 240 horsepower from 5000 to 6500 rpm. Visualize two straight lines with a downward kink at 5000 rpm, and there's your torque curve.

That's what you get on an engine dyno, not a chassis dyno. Due to differences in the way the two dyno types load engines (particularly turbocharged ones), you'll never see that much torque so low in the rev range on a chassis dyno.

>But this new turbo engine is still a healthy thing, exhibiting a nice, low boost threshold and a surprisingly linear nature. You have to intentionally drive like a tool to make it feel laggy. Don't drive like a tool. Problem solved.

Here's what we found on MD Automotive's Dynojet chassis dyno:

The Z4 churns out a usefully ample slice of torque from down low, reaching a peak of 249 lb-ft and remaining north of 200 lb-ft clear out to 5700 rpm. By the time you encounter the 7000 rpm rev limit, the party's well over and you should have shifted a couple hundred rpm ago.

That curious hole between 5300 to 5800 rpm was no fluke, as it was evident in every one of the dozen or so pulls I performed. There was no audible detonation at any point in this dyno exercise, but it's possible that timing is being pulled here due to our crappy 91 octane premium fuel. That's speculation, of course. In any case, were the hole filled in, peak power would have risen a bit from its observed 233 horsepower.

Boost comes seamlessly and with the faintest trace of turbo whine, and if you listen closely you can hear the compressor bypass valve make a little chuff when you lift the throttle. It makes just enough turbo-related sound to give it character, and not so much that anyone will mistake this for a boy-racer ride. 

Looks good so far, but how does this German 2.0-liter turbocharged and direct-injected four cylinder compare to another, better-known German 2.0-liter turbocharged and direct-injected four cylinder? I'm glad you asked.

Simply put: if you like the GTI, you'll love the Z4. The character is similar, there's just more soup in the BMW's jar.

The Volkswagen sounds way better, though -- somehow, this Z4 sounds lazy and a bit trucky, like it's been hanging out with a Viper V10 too much. Shame about that part. Indeed, it might be the sole trait in which BMW's erstwhile straight six holds an edge over the new turbo four.